MUSCA THE FULCRUM 39 



haustellum. The palps, which are composed of a single joint, are attached 

 to its anterior surface; when the proboscis is extended for use they are 

 directed straight forward. Within the rostrum there is a strong chitinous 

 structure known as the fulcrum, which contains the pharynx of the 

 fly; this occupies the greater part of the rostrum. The pharynx 

 within it is connected with the food canal in the haustellum by means 

 of a small chamber, the homologue of the buccal cavity in Tabanus. 

 In addition to these structures, the rostrum contains certain important 

 muscles and air sacs, and the salivary duct. 



The fulcrum (Plate VIII, fig. 1), to use Kraepelin's simile, resembles a 

 Spanish stirrup iron ; the foot plate being posterior, and the toe at the lower 

 end. The posterior plate is oblong, concave forwards in both diameters to 

 a slight extent, and produced at the upper end into a pair of conspicuous 

 lateral cornua. At the lower end it is narrowed in the transverse diameter 

 and broadened out a little in the antero-posterior, so that it appears some- 

 what funnel-shaped. The lateral plates, corresponding to the high sides 

 of the stirrup, are triangular, and are continuous with the sides of the 

 posterior plate. Their free superior and anterior sides are deeply incur- 

 ved, the curvature on the latter being interrupted by a sharp spine, which 

 projects downward and forward in the lower third. These two lateral 

 sides are connected with one another in front by a transverse arch, which 

 passes between their anterior angles ; this corresponds to the part to 

 which the stirrup leather would be attached. It is roughened and 

 thickened for muscle attachment. The anterior arch of the fulcrum is 

 continuous with the membrane which forms the anterior wall of the rost- 

 rum, and can be seen in fresh specimens when the proboscis is extended. 



The posterior plate of the fulcrum, as above described, is the ven- 

 tral wall of the pharynx. Anterior to it there is a thinner plate of 

 a corresponding size, which, in the resting condition, is closely ap- 

 posed to it, and forms the dorsal wall of the cavity (Plate X, fig. 4). It is 

 traversed down the middle line by a thick and rough ridge, to which most 

 of the dilator muscles are attached. At the upper end the two walls of 

 the pharynx merge to form the oesophagus, while at the lower end they 

 are connected with the walls of the buccal cavity by a short membrane. 

 The space between the lateral walls of the fulcrum is occupied by the 

 dilator muscles of the pharynx, which pass from the anterior arch and 

 the adjacent membrane to the ridge in the middle line of the dorsal wall 

 of the pharynx. When these muscles contract the two plates are drawn 

 apart, and the fluid food is drawn up into the cavity of the pharynx, 

 exactly as in Tabanus. 



